1. Field of the Invention
The present invention relates to a liquid crystal display device, and in particular to a transflective liquid crystal display incorporating a transflective reflector inside a liquid crystal element thereof, capable of effecting bright display in black and white or in color in the case of either reflective display utilizing external light or transmissive display by backlighting.
2. Description of the Related Art
For a conventional reflection-type liquid crystal display device, there is in use mainly a liquid crystal display device of a constitution wherein a TN (twisted nematic) liquid crystal element or an STN (supertwisted nematic) liquid crystal element is disposed between a pair of polarizing films, and a reflective layer is installed on the outside of one of the polarizing films.
With the reflection-type liquid crystal display device of such a constitution, external light entering through one of the polarizing films from the visible side of the device is either absorbed by the other of the polarizing films or transmitted therethrough and reflected by the reflective layer installed on the outside thereof, going out towards the visible side after passing through again the liquid crystal element and the pair of the polarizing films, thereby effecting reflective display, depending on whether or not the direction of polarization of the external light is rotated when passing through the liquid crystal element.
That is, the external light entering from the visible side passes through two sheets of the polarizing films before reaching the reflective layer, and reflected light of the external light goes out towards the visible side after passing through again the two sheets of the polarizing films, thereby effecting white display, so that magnitude of light attenuation by the agency of the polarizing films increases, resulting in deterioration of brightness of images in display.
Moreover, since the reflective layer is installed on the outside of a glass substrate of the liquid crystal element, there arises a problem that shadows come to appear on display.
To cope with these problems, a single polarizing film reflection-type liquid crystal display device, capable of effecting display with just one sheet of polarizing film, has since been proposed. With such a liquid crystal display device having one sheet of polarizing film, brightness of images in display can be improved in comparison with that for the case of the conventional reflection-type liquid crystal display device employing the two sheets at the polarizing films. Further, with the single polarizing film reflection-type liquid crystal display device, a reflective layer is formed inside a liquid crystal element, thereby enabling the problem of the shadows appearing on display to be solved.
Such a single polarizing film reflection-type liquid crystal display device is composed of one sheet of polarizing film, one sheet of retardation film, and at liquid crystal element incorporating a reflective layer, as disclosed in, for example, JP, 04-97121, A. Further, a single polarizing film reflection-type liquid crystal display device employing an optical compensatory element having a structure twisted in the direction opposite to the twist direction of a liquid crystal layer in place of a retardation film is also disclosed in, for example, JP 10-123505, A.
With such conventional single polarizing film reflection-type liquid crystal display devices as described above, however, it is not possible to install a backlight because the reflective layer does not allow light to pass therethrough so that it has not been possible to see display at places where external light is dim or at night.
Accordingly, there has been developed a transflective liquid crystal display device, employing a transflective layer serving as a half-mirror, made up of a very thin aluminum film with thickness in a range of 0.01 to 0.03 xcexcm, formed by the vapor deposition method or the sputtering method as a reflective layer, or employing a transflective layer provided with an opening every pixel by use of photoetching method as a reflective layer. As a result, display can be effected by lighting up a built-in backlight of the liquid crystal display device at places where external light is dim or at night.
However, in the case of using a thin metal film for the half-mirror, significant variation in transmittance of the transflective layer occurs depending on the thickness thereof, and there will be an increase in fluctuation of transmittance as well as reflectance of the transflective layer at the time of production. For these reasons, such a transflective liquid crystal display device as described has a drawback in that large dispersion will occur in brightness of images in the case of reflective display utilizing external light, and in luminance in the case of transmissive display by backlighting.
A liquid crystal display device employing a transflective layer provided with an opening for every pixel has been disclosed in, for example, JP, 10-282488, A.
However, with such a liquid crystal display device as described above, a reflective layer made up of an aluminum film is formed on top of a first substrate 1 composing a liquid crystal element, and an opening 29 is provided in regions of the reflective layer, corresponding to respective pixels, thereby forming a transflective 27 as shown in FIG. 12. The transflective layer 27 has a thickness in the order of 0.1 to 0.2 xcexcm, and even after a planarization treatment is applied thereto by providing a protective film (top coat layer) 8, the surface of the protective film 8 and the surface of first electrodes 3 formed on top of the protective film 8 are left with differences in level of 0.05 xcexcm or more.
Due to the differences in level, there occur a difference of 0.05 xcexcm or more between cell gaps, which are gaps holding a liquid crystal layer sandwiched between the first substrate 1 and a second substrate (not shown) in-between, namely, between those opposite to the respective openings 29 of the transflective layer 27, and those opposite to regions thereof, other than the respective openings 29. As a result, there have been encountered cases where display unevenness, and in the worst case, alignment defect have occurred thereby degrading display quality considerably. Particularly, in the case of using and STN liquid crystal element having a twist angle in a range of 180 to 260xc2x0, there is the need for strictly controlling the cell gaps, however, in such a case, it becomes difficult to implement controlling the same, so that display unevenness tends to occur due to the difference between the cell gaps, and further, there have arisen even cases where alignment defect has occurred due to the induction domain typical of STN liquid crystal during a period of applying a driving voltage.
It is therefore an object of the invention to solve the problems described above, encountered by conventional liquid crystal liquid devices of various types and to provide a transflective liquid crystal display device capable of effecting blight reflective display utilizing external light and transmissive display by backlighting, and having less display unevenness and less alignment defect with little fluctuation in display brightness.
To this end, the transflective liquid crystal display device according to the invention comprises a liquid crystal element composed of liquid crystal sandwiched between a first substrate and a second substrate, and a transflective layer installed on the inside of the first substrate, wherein the transflective layer is a thin metal film having transparent portions formed by means of anodic oxidation.
The transflective liquid crystal display device preferably further comprises a first polarizing film disposed on the outside of the second substrate of the liquid crystal element, a second polarizing film and a backlight, disposed in sequence on the outside of the first substrate.
As a result, an untransparent portion and transparent portion of the transflective layer have substantially, the same thickness, and cell gaps in which the liquid crystals are sandwiched between the first substrate and the second substrate of the liquid crystal element are rendered uniform. Accordingly, occurrence of display unevenness and alignment defect is presented, so that bright display without unevenness and in good contrast can be effected in the case of transmissive display by backlighting as well as reflective display utilizing external light.
Further, it is desirable that the transflective layer and first electrodes are installed on the inner face of the first substrate, second electrodes are installed on the inner face of the second substrate, nematic liquid crystal of twisted alignment are used as the liquid crystal, a first optical compensatory element is disposed between the second substrate and the first polarizing film, and a second optical compensatory element is disposed between the first substrate and the second polarizing film.
Supertwisted nematic liquid crystal having a twist angle in a range of 180 to 260xc2x0 may be used for the nematic liquid crystal.
With these features, pits and projections are preferably provided on the surface of an untransparent portion of the transflective layer, thereby rendering to form a scattering layer. Or a light scattering layer may be installed on the outside of the second substrate of the liquid crystal element.
The first optical complementary element can be composed of one sheet of retardation film, or a plurality of sheets of retardation films. Otherwise, the first optical compensatory element may be composed of one sheet of twisted retardation film, or a twisted retardation film and one sheet of retardation film or a plurality of sheets of retardation films.
Color display can be effected by installing color filters in a plurality of colors on either the first substrate or the second substrate of the liquid crystal element.
A thin aluminum film provided with transparent portions made of aluminum oxide is preferably employed as the transflective layer. Further, an oxide film formed by anodic oxidation is preferably provided on the untransparent portion of the transflective layer.
Since crossover points of the first electrodes and the second electrodes, opposed to each other, respectively, inside the liquid crystal element, constitute respective pixels, the transparent portions of the transflective layer are desirably provided at positions corresponding to the respective pixels. Each of the transparent portions of the transflective layer, formed in a slit shape, may be provided at positions corresponding to a plurality of pixels in succession, respectively.
An area ratio of the transparent positions to the transflective layer is preferably in a range of 5 to 30%, and in particular, the area ratio in a range of 10 to 25% is desirable.
By installing a protective film formed of a transparent and insulating material between the transflective layer and the first electrodes, provided on the first substrate of the liquid crystal element, the surface of the transflective layer can be rendered more flatter, and the transflective layer can be insulated from the first electrodes.
The above and other objects, features, advantages of the invention will be apparent from the following detailed description which is to be read in conjunction with the accompanying drawings.